Conventionally, electrical power has usually been stored in batteries. Another device for storing energy is a capacitor, and more recently the so-called supercapacitor. Very substantial efforts have been made to develop improved capacitors for storing electrical energy.
The requirement for capacitance is the ability to separate charge at a specified potential. The prototypical capacitor consists of two metal plates, with a potential difference between the plates. In the charged state, one plate will have a net positive charge, the other a net negative charge. The capacitance can be determined from the area of the plates and the separation between the plates. Placing a solid dielectric material between the plates increases the capacitance, as the same potential difference between the plates leads to larger net charge on each plate.
Recent developments in capacitor technology have led to replacement of the metal plates with high surface area conductive materials, such as carbon, and replacement of the solid dielectric with a liquid electrolyte. In case of carbon electrodes, the capacitance arises from the double layer mechanism, where the ions in the electrolyte move adjacent to the electrode surface. In this case, the capacitance increases due to two factors, the increase in the area of the electrode due to the porosity, and the decrease in the charge separation distance.
The recent developments in synthesis of high surface area materials have also led to the development of capacitors based on a second mechanism, the so-called faradaic capacitors. The faradaic capacitors are composed of a solid state electrode with a liquid electrolyte. The operation principle of these capacitors is based on reversible reactions at the interface at certain potential. There are different characteristics of the second type of capacitors; the charge transfer reaction occurs at the interface of the outer porous layer, the substrate (current collector) is a different material than the external layer. The ions are integrated in the structure of the high surface area material (commonly an oxide or nitride) by reacting either by substitution or by integration of the ion within the structure of the material. To cite just one example, see Piao et al. “Intercalation of Lithium Ions into Graphite Electrodes studied by AC Impedance measurements,” J. Electrochem Soc. 146, 2794-2798 (1999). The stability will depend on the reversibility of this reaction (or process). If the reduction or oxidation process consumes more species than the reversible reaction, or if there is another species formed at the surface, the reversibility is modified.
Recently, a third type of capacitor, the “hybrid” capacitor has also been reported. In this capacitor, both the double-layer and the faradaic mechanism are used, to provide enhanced capacitance, and to take advantage of operational advantages of each mechanism.
A liquid electrolyte is either aqueous, with a high concentration of acid, base, or salt, or non-aqueous with a salt dissolved in an organic or inorganic solvent. There are a wide variety of solvents and salts available for such use, offering specific advantages depending on the application being considered (e.g., low temperature vs. high temperature). Ionic liquids based on the imidazolium cation have recently received attention as nonaqueous electrolytes in various electrochemical devices (Koch et al., J. Electrochem. Soc. 143:155, 1996). These electrolytes have significant advantages compared to the numerous quaternary onium salts that have been previously investigated for use in carbon double-layer capacitor.
Electrochemical capacitors based on nonaqueous electrolytes offer greater electrochemical stability (up to 4 V) compared to aqueous systems (limited to approximately 1V), thereby providing greater energy storage (E=½CV2). However, due to the lower conductivity of nonaqueous electrolytes compared to aqueous systems, lower power capabilities are observed. In addition, with the porous materials used in electrochemical capacitors, the high viscosity typically associated with the high dielectric constant solvents used in nonaqueous electrolytes is detrimental to conductivity in porous electrodes. Furthermore, the lower ion concentrations typically obtained with nonaqueous electrolytes result in increased electrolyte volume requirements for packaged devices.
A solid state electrode can be composed of a nanoporous transition metal compound placed on a high surface area conductive medium, such as carbon black, or carbon nanotube (CNT) films, combined with a binder to ensure physical integrity. If the ions move into the transition metal compound, the capacitance mechanism is faradaic, or possibly hybrid, while if the ions do not enter the transition metal compound the mechanism is purely double layer.
There are numerous reports in the prior art describing methods of forming electrodes from composites of carbon and metal oxides or mixed metal oxides. For example, Leela Mohana Reddy et al. in “Asymmetric Flexible Supercapacitor Stack”, Nanoscale Research Letters, Volume 3, Number 4/April, 2008, describe the preparation of a supercapacitor with metal oxide and multiwalled carbon nanotubes (MWNTs) composites synthesized by a sol-gel method. Fan et al. in “Preparation and capacitive properties of cobalt-nickel oxides/carbon nanotube composites”, Electrochim. Acta, 52 (2007) 2959, reported the preparation of nickel-cobalt oxides/carbon nanotube (CNT) composites. Kuan-Xin et al. in “Electrodeposition of Nickel and Cobalt Mixed Oxide/Carbon Nanotube Thin Films and Their Charge Storage Properties,” J. Electrochem. Soc., 153, A1568-A1574 (2006) reported a method of electrochemically depositing a mixed metal oxide on a film of carbon nanotubes.
In U.S. Pat. No. 5,079,674, Malaspina describes a composite supercapacitor made from metal oxide and carbon black. In his method, carbon black is added to a solution of the metal salt, converted to its hydroxide or oxide, a fluorocarbon polymer added, and the resulting material is converted to sheet form and dried in an oven at a temperature of between about 80° C. and 125° C. The resulting sheet material is laminated to a separator, cut into a desired shape, and assembled to form a supercapacitor. Malaspina does not provide specific examples or capacitance data; and there is no description of the effect of synthetic conditions on material properties.
Yoon et al. in “CoNi Oxide/Carbon-Nanofiber Composite Electrodes for Supercapacitors”, Int. J. Electrochem. Sci., 3 (2008) 1340-1347, report the synthesis of cobalt-nickel oxide/VGCF (vapor grown carbon fiber) composites for super capacitors. In this method, a weighed quantity of VGCF was added to a cobalt-nickel nitrate solution, sonicated for 1 hour and then dropped onto a nickel foam and annealed at 250° C. for 2 hours. Yoon et al. reported that the cobalt-nickel oxide/VGCF composite electrode exhibited a peak specific capacitance value of 1271 Fg−1 at a scan rate of 5 mV·s−1, however neither the weight of the nickel foam substrate nor the weight of the VGCF was included in the specific capacitance calculations. The 3-dimensional nickel foam substrate has advantages over the more typical 2-dimensional metallic foil type of current collector, including providing a very high surface area for greater capacitance, but has disadvantages due to its cost, large volume and weight.
Despite extensive research and development, there remains a need for improved capacitors for the storage of energy.